The present invention relates to a variable displacement compressor that forms a refrigerating circuit of, for example, a vehicle air conditioner, and more particularly, to a controller for controlling displacement of a variable displacement compressor.
The refrigerant circuit of a typical air conditioner includes a gas cooler, an expansion valve, which functions as a depressurizing device, an evaporator, and a compressor. The compressor draws in refrigerant gas from the evaporator, compresses the refrigerant gas, and discharges the compressed gas to a gas cooler. The evaporator functions to perform heat exchange between the refrigerant flowing through the refrigerant circuit and the air in the passenger compartment. The heat transferred from the air that passes by the vicinity of the evaporator to the refrigerant flowing through the evaporator is in accordance with the level of the heating load or cooling load. Accordingly, the pressure of the refrigerant gas at the outlet and downstream side of the evaporator reflects the level of the cooling load in addition to the ambient temperature of the evaporator.
Variable displacement swash type compressors are often installed in automobiles. Such a compressor incorporates a displacement control mechanism that either maintains the ambient temperature of the evaporator at a predetermined target value (temperature setting) or maintains the pressure at the outlet of the evaporator (suction pressure) at a predetermined target value (suction pressure setting). To adjust the flow rate of the refrigerant in accordance with the cooling load, the displacement control mechanism feedback controls the displacement of the compressor, or the inclination angle of the swash plate, using the ambient temperature of the evaporator or the suction pressure as a control index.
A typical displacement control mechanism is a control valve referred to as an internal control valve. The internal control valve senses the suction pressure with a pressure sensing member, such as a bellows or a diaphragm. The pressure sensing member moves in accordance with the suction pressure. This, in turn, moves a valve body and adjusts the open amount of the valve. Accordingly, the internal control valve adjusts the pressure (crank pressure) of a swash plate chamber (crank chamber) so as to determine the swash plate angle.
A simple internal control valve using only one suction pressure setting cannot finely control the air conditioner. Japanese Laid-Open Patent Publication No. 10-318418 describes an example of a variable suction pressure setting control valve that solves this problem. An external device electrically controls this control valve to vary the suction pressure setting. The variable suction pressure setting control valve is formed by combining the above-described internal control valve with an actuator such as an electromagnetic solenoid that electrically adjusts an urging force. Accordingly, the variable suction pressure setting control valve is externally controlled to vary mechanical spring force that is applied to the pressure sensing member to determine the suction pressure setting of the internal control valve.
In the variable suction pressure setting control valve, when the actual suction pressure is not included in the range of the variable suction pressure setting (i.e., the range in which the suction pressure setting may be set), the valve body does not move even if the actual suction pressure changes or even if the suction pressure setting changes. For example, cool-down (rapid cooling) may be started in a state in which the actual suction pressure is greater than the variable suction pressure setting range. In such a case, the displacement of the compressor remains maximum until the actual suction pressure falls into the variable suction pressure setting range. The discharge pressure of the compressor increases when the compressor operates in the maximum displacement state. If the actual suction pressure is much greater than the variable suction pressure setting range when cool-down is started due to a high heating load or other reasons, the operation of the compressor in the maximum displacement state is prolonged. This excessively increases the discharge pressure.
Instead of using the above-described variable suction pressure setting control valve to control the displacement of the variable displacement compressor, a pressure sensor for detecting the suction pressure or a temperature sensor for detecting the ambient temperature of the evaporator may be used. More specifically, an external device controls the open amount of a control valve, which is an electromagnetic valve (electromagnetic actuator and valve body), so that the pressure detected by the pressure sensor becomes equal to the suction pressure setting or so that the temperature detected by the temperature sensor becomes equal to a predetermined temperature setting. In this case, however, the operation of the compressor in the maximum displacement state is also prolonged when the pressure detected by the pressure sensor is much greater than the suction pressure setting or when the temperature detected by the temperature sensor is much greater than the temperature setting.
Therefore, when controlling the displacement of the variable displacement compressor to adjust the cooling load by maintaining the ambient temperature of the evaporator at the temperature setting or by maintaining the suction pressure at the suction pressure setting, the discharge pressure may be excessively increased regardless of whether the control valve is controlled by an internal autonomous device or an external device.
Excessive increase of the discharge pressure affects the durability of each device and pipe in the refrigerant circuit. The refrigerant circuit normally includes a pressure relief valve (PRV). The PRV releases refrigerant out of the refrigerant circuit when the discharge pressure excessively increases, such as when a device does not function properly. In this manner, the PRV protects normally functioning devices and pipes. However, the PRV may be activated even though the compressor is functioning properly. In such a case, troublesome work, such as charging refrigerant, would be required for subsequent air-conditioning.
The discharge pressure is especially increased when using carbon dioxide as the refrigerant in comparison to when using, for example, FREON as the refrigerant. In this case, since the tolerance margin with respect to durability for the compressor and the pipes are small, the PRV has a tendency of being activated. Further, the critical temperature of the carbon dioxide refrigerant is low. Thus, the carbon dioxide refrigerant may be in a critical state when the ambient temperature is high, such as during the summer. In such a state, the discharge pressure of the carbon dioxide refrigerant tends to increase more suddenly and excessively, compared to a liquid refrigerant, when the compressor is operated in the maximum displacement state. Thus, the PRV would also have a tendency of being activated in this state.
When using, for example, a suction pressure setting variable control valve to control the displacement of a variable displacement compressor, the maximum value of the variable suction pressure setting range may be increased to solve the above problem. This would readily decrease the actual suction pressure to the variable suction pressure setting range without prolonging the operation of the compressor in the maximum displacement state during cool-down. If the actual suction pressure is in the variable suction pressure setting range, the sensing member functions to decrease the displacement of the compressor. This suppresses excessive increase of the discharge pressure.
However, the suction pressure is much higher when using a carbon dioxide refrigerant in comparison to when using a FREON refrigerant. Accordingly, when using a carbon dioxide refrigerant, the sensing member must be much smaller than that used for a FREON refrigerant to obtain the same displacement control characteristics. Nevertheless, it is presently difficult to make the sensing member more compact. For this reason, it is difficult to further widen the range of the variable suction pressure setting when using a carbon dioxide refrigerant.